The present invention relates generally to heating, ventilation and air conditioning (HVAC) systems for vehicles.
The HVAC systems used in conventional vehicles are typically powered by the engine, which is continuously running, and so are not impacted by the power drain on a battery pack. Such systems are based upon the fact that a conventional (non-hybrid) vehicle's engine coolant temperature is controlled to a somewhat constant temperature, using an engine thermostat, and that the heater core coolant flow rate varies with engine speed. These HVAC systems when operating in heating modes typically adjust the position of a temperature door to achieve the desired temperature of the air flowing into a passenger compartment.
However, for plug-in hybrids & electric vehicles, an important vehicle performance objective is vehicle range in the pure electric-vehicle mode. Such extended range electric automotive vehicles use a motor, powered by a battery, for moving the vehicle, with an engine or fuel cell used as a kind of on-board generator to recharge the battery pack. Some types of hybrid vehicles (such as plug-in hybrid vehicles) also operate for extended periods in electric only modes. In electric vehicle mode, there is no engine heat rejection and so battery pack energy is consumed in order to power the accessories. The electric only driving range of automotive vehicles, with battery powered electric motors providing the motive force, can be greatly reduced by vehicle electric accessory loads. Some of the highest electric accessory loads are used to provide heat to the passenger compartment of the vehicle for windshield defrost/defog and occupant comfort. Thus, minimizing the electric power consumption for HVAC systems can greatly improve the electric only driving range of these vehicles, as well as sometimes improve the total driving range of the vehicles.
Moreover, for fuel cell vehicles, the maximum fuel cell coolant temperature is limited to a lower level than with an internal combustion engine, so supplemental heat may be required to provide the desired heat to the passenger compartment for defrost and warming functions—especially when the vehicle is operating in low ambient air temperature conditions.